This invention relates to a developing device for an electrophotographic copier or a laser printer for developing an electrostatic latent image formed on a recording medium.
An electrophotographic copier or the like produces a visible image from an electrostatic latent image formed on a recording medium by attaching toner made of color pigments to this electrostatic latent image. In order to produce an image in different colors instead of a simply black and white image, there are image formation apparatus such as copiers provided with a plurality of developing devices each containing therein toner of a different color such that these developing devices can be used selectively. A copier adapted to produce a full-color image may be provided, for example, with individual developing devices containing cyanic, magenta and yellow toner and, if necessary, with a developing device with black toner. The individual developing devices develop electrostatic latent image, separately formed on recording media through filters of different colors, by applying toner of these colors. It is very important, therefore, to design these developing devices such that toner therefrom is prevented from getting attached to the electrostatic image when they are not being used for development.
According to a method which has been used commonly for this purpose, the gap between the recording medium and the developing device is controllably changed such that, when the developing device is not used, toner therefrom will not be supplied to the developing region. With an image formation apparatus having a developing device thus structured, however, fluctuations tend to appear in conditions of development, causing changes in image densities and the like. If the gaps cannot be determined accurately, normal images cannot be obtained consistently.
For the case of a developing device using a magnetic brush, there have been attempts to displace the magnet in the developing device such that the magnetic brush is prevented from contacting the recording medium. Displacements of the magnet, however, tend to affect the image density. In the case of a developing device using a two-component developing agent, furthermore, the carrier can still be attached to the recording medium to significantly reduce the image quality.
In view of the above, a new idea has been developed according to which a stirrer roller is provided in the supply route of developing agent to the magnetic roller opposite to the recording medium and a cylindrical shutter having slits is engaged coaxially around its outer periphery. Not only is this shutter rotated controllably to open and close the supply route, used developing agent is picked up by the magnet roller for the development and is further transported by another magnet roller to a place where stirring is taking place. With a developing device thus structured, the toner therein can be prevented from attaching to an electrostatic image on the recording medium without varying the gap between the developing device and the medium. In addition, a sensor for detecting toner concentration may be provided in the toner supply route between the shutter and the magnet roller for development such that the toner concentration of the developing agent as it is delivered to the magnet roller can be maintained at a uniform level.
If the shutter is closed and the toner supply route is blocked, however, the magnet roller does not form a magnetic brush any more and the sensor cannot detect the toner concentration. As a result, since the magnetic brush remains formed only for a brief period of time, there is not enough time to supply toner sufficiently even if the sensor detects a low toner concentration. In the case of a full-color development, in particular, the developing time for each color, or the time during which a magnetic brush is formed, is extremely short, being only about two seconds. Thus, even if the sensor determines that the toner concentration is low, it is extremely difficult to supply a required amount of toner within such a short period of time. If toner is not sufficiently supplied, not only does the image density become low but also the carrier comes to be attached to the recording medium to adversely affect the image quality.